1. Field of the Invention
The present invention relates to a sheet conveyance apparatus for a printer, a facsimile machine, a copying machine, or a multifunction peripheral having a plurality of functions, and further relates to an image forming apparatus including the sheet conveyance apparatus.
2. Description of the Related Art
There are various image forming apparatuses, including an electrophotographic type, an offset printing type, and an inkjet type, which are conventionally used. For example, a conventional electrophotographic color image forming apparatus includes a plurality of photosensitive drums disposed in a straight line (referred to as “tandem type”) or disposed along a circular path (referred to as “rotary type”).
Among conventionally used image transfer methods, a method for directly transferring a toner image from a photosensitive drum to a sheet is referred to as a “direct transfer method”. A method for transferring a toner image from a photosensitive drum to an intermediate transfer member and then transferring the toner image from the intermediate transfer member to a sheet is referred to as an “intermediate transfer method.”
Compared to the offset printing machines, recent electrophotographic image forming apparatuses are advantageous in not requiring printing plates and are preferably used for Print On Demand (POD) services, according to which a small amount of printing can be flexibly performed. However, to attain expected task goals, image forming apparatuses dedicated to the POD services are required to perform high performances suitable for the POD services. In this respect, accuracy in positioning an image on a sheet is an important factor to be satisfied. For example, in an image forming apparatus configured to perform two-sided printing, the image positioning accuracy includes an accuracy of positional adjustment between images formed on front and reverse pages.
The position in a sheet conveyance direction, the position in a direction perpendicular to the sheet conveyance direction, a magnification rate of the image, and a skew amount of the sheet are example factors influencing the position of an image formed on a sheet. Thus, eliminating differences in these factors is a key to attain a satisfactory level of positioning accuracy.
For example, an image forming apparatus can perform electrical control to eliminate differences in the sheet conveyance position and the magnification of an image. However, correcting the skew of a sheet using electrical control is difficult. For example, to correct the position of a conveyed sheet, the apparatus can electrically control irradiation timing/position of a laser beam based on an image signal supplied to a photosensitive drum. For example, to correct the magnification of an image, the apparatus can electrically control the irradiation range of the laser beam emitted to the photosensitive drum.
On the other hand, to correct the skew of a sheet, electrically detecting a skew amount of a conveyed sheet and electrically forming an inclined image matching the inclined sheet so as to correct the position of an image relative to the sheet is feasible. However, when an image forming apparatus can adjust the inclination of an image for each sheet while forming a color image with three or four colors overlapping each other, deviations of respective colors in dot formation may change the tint of an image on each sheet depending on a skew amount of the sheet. Furthermore, a relatively long time is required to calculate an inclination of the image. Therefore, productivity of the apparatus decreases greatly. Thus, an appropriate mechanism or device for mechanically correcting the skew of a sheet is required.
The skew correction mechanisms are roughly classified into the following types (or groups).
A skew correction mechanism belonging to a general type includes a pair of registration rollers disposed on the upstream side of a transfer unit, which can eliminate a skew amount of a conveyed sheet (conveyed transfer material) by causing the leading edge of the sheet to collide against a nip portion of the registration rollers in a stopped state. This type of skew correction mechanism excessively conveys a sheet after the leading edge of the sheet reaches the nip portion of the registration rollers. Therefore, while the conveyed sheet deforms into a loop shape, the leading edge of the sheet can be aligned along the nip portion of the registration rollers so as to eliminate a skew amount.
A skew correction mechanism belonging to another type includes a calculation unit configured to calculate a skew amount of a sheet based on a detected inclination of the leading edge of the sheet and two independently driven rollers disposed in a direction perpendicular to the sheet conveyance direction. This type of skew correction mechanism independently changes the conveyance speed of each driving roller according to a calculated skew amount of a sheet, thereby causing the sheet to rotate in a predetermined direction to eliminate the skew.
Furthermore, a skew correction mechanism belonging to yet another type includes a reference surface extending along the sheet conveyance direction and skew rollers obliquely conveying a sheet toward the reference surface. The reference surface causes a conveyed sheet to change its orientation (reduce a skew amount) while regulating the side edge of the conveyed sheet.
An example skew correction mechanism configured to correct the orientation of a sheet while regulating the side edge of the sheet with the reference surface is described below with reference to the drawings.
FIGS. 23A and 23B illustrate a skew correction unit as seen from a sheet conveyance direction, according to which a sheet moves from the front side to the rear side of the drawing. The skew correction unit includes a skew correction roller 32 and a pressing roller 34, which can cooperatively hold a sheet S and obliquely convey the sheet S to a reference surface 311 of a reference guide unit 31. After the sheet S collides against the reference surface 311, the skew correction roller 32 and the pressing roller 34 cause the sheet S to rotate to change its orientation (reduces a skew amount) and start moving straight along the reference surface 311.
As illustrated in FIG. 23A, when a side edge of the sheet S is obliquely conveyed between the skew correction roller 32 and the pressing roller 34, the sheet S is guided by an upper guide 312 and a lower guide 313 of the reference guide unit 31. The upper guide 312 and the lower guide 313 prevent the sheet S from buckling. The method for correcting the skew of a sheet by causing a side edge of the sheet to change its orientation along a reference surface is advantageous in the following points.
When an image forming apparatus performs image formation processing on front and reverse sides (first and second pages) of a sheet, the image forming apparatus performs a switchback operation to switch leading/trailing edges of the sheet for the first and second pages. In this case, the apparatus does not switch the side edges of the sheet. The apparatus performs the skew correction on the first and second pages of a sheet similarly at the same position in a direction perpendicular to the sheet conveyance direction. Therefore, the skew correction method using a reference surface can accurately set a start position of an image relative to a side edge of a sheet. The apparatus can perform two-sided image formation processing without causing any deviation between images on the front and reverse sides of a sheet.
According to a method for performing skew correction at the leading edge of a sheet, a deviation between images on the first and second pages cannot be corrected if the deviation is caused in a direction perpendicular to the sheet conveyance direction. Namely, even if the skew correction ability is high, images formed on the front and reverse sides of a sheet may deviate relative to a side edge of the sheet.
In the POD market, image forming apparatuses are required to perform image formation on various types of recording materials, including plain paper different in grammage (e.g., not less than 40 g/m2 and not greater than 350 g/m2), coated sheet, film, and other special materials.
As described above, a representative skew correction method includes conveying a sheet obliquely toward a reference surface to cause the conveyed sheet to collide at its side edge against the reference surface and change its orientation so as to reduce a skew amount of the sheet. However, recent image forming apparatuses are required to use various types of sheets different in thickness and material. If a conveyed sheet is thin or made of a material having a lower stiffness, the sheet may buckle when it collides against the reference surface. As illustrated in FIG. 23B, if the sheet S has a lower stiffness, the sheet S may buckle in a clearance between the upper guide 312 and the lower guide 313 when the sheet S collides against the reference surface 311.
In this case, the skew correction cannot be performed accurately and accuracy in positioning an image on a sheet deteriorates correspondingly. Furthermore, paper jam may occur due to buckling of a sheet. The side edge of a sheet may be broken or damaged. In general, the clearance between the upper guide 312 and the lower guide 313 is set to be larger than the thickness of a thickest sheet processed by the image forming apparatus. Therefore, the clearance between the upper guide 312 and the lower guide 313 is not sufficiently narrow to prevent a thin sheet from buckling.
Hence, to surely convey a sheet while guiding a side edge of the sheet along the reference surface without causing any buckling, an apparatus discussed in Japanese Patent Application Laid-Open No. 2002-356250 includes a mechanism for adjusting the clearance between upper and lower guides according to the thickness of a sheet. The discussed conventional apparatus is operative to decrease the clearance between the upper and lower guides when the conveyed sheet is a thin sheet (i.e., a sheet having a lower stiffness). Therefore, the apparatus can surely guide the side edge of a sheet along the reference surface while preventing the sheet from buckling.
However, according to the above-described conventional apparatus configured to adjust the clearance between the upper and lower guides according to the thickness of a sheet, a detection unit is required to operate accurately. The detection unit is, for example, a contact-type sensor or a reflection-type optical sensor capable of directly detecting the thickness of a sheet. Another detection unit can detect the thickness of a sheet based on the displacement of a conveyance roller movable when it nips the sheet.
However, if the detection by such a detection unit is performed while a sheet is continuously conveyed and not stopped, a significant amount of detection error (e.g., approximately 10%) arises due to an up-and-down movement of the conveyed sheet and an eccentricity of each conveyance roller in addition to inherent errors caused by an individual sensor. Moreover, according to the method for detecting the thickness of a sheet based on a displacement amount of a conveyance roller movable when it nips a sheet, accurately detecting the thickness of a thin sheet is difficult because the displacement of the roller is small.
Furthermore, there is a method for adjusting the clearance between upper and lower guides based on sheet thickness information directly entered by a user, instead of automatically detecting the thickness of a sheet. In this case, a user is required to enter the sheet thickness information and may erroneously set the information.
Furthermore, compared to the thickness of a sheet, the stiffness of a sheet is a decisive factor to prevent a sheet from buckling when the sheet collides against a reference surface. FIG. 22 is a graph illustrating plots representing various types of sheets with respect to a relationship between the thickness of a sheet and the stiffness of the sheet. As understood from the data illustrated in FIG. 22, the stiffness of a specific type of sheet is greatly different from the stiffness of another type of sheet even if these sheets are similar in thickness.
Furthermore, as understood from the graph illustrated in FIG. 22, there is a tendency that the stiffness of a thin sheet greatly decreases if the thickness is slightly changed. Thus, according to the method for adjusting the clearance between the upper and lower guides simply based on the thickness of a sheet, it is difficult to prevent the sheet from buckling. Therefore, if the stiffness of a thick sheet is low, it is required to narrow the clearance between the upper and lower guides to prevent the sheet from buckling. However, the above-described conventional apparatus cannot prevent a thick sheet from buckling if the sheet has a lower stiffness, because the apparatus does not change the guide clearance based on the stiffness of a sheet. Moreover, as a specific mechanism for adjusting the clearance between upper and lower guides, a driving unit configured to drive a motor and a control unit configured to control the driving unit are required. Therefore, the cost for the apparatus increases.